VPCLIP arc options, visible linked clip boundaries, XCLIP depth rejection

- VPCLIP arc mode takes Angle, CEnter, Direction, Radius and Second pt with
  the reference prompts, and CLose closes with a tangent arc; the arc math
  is shared with PLINE.
- Viewport clip boundaries from MVIEW and VPCLIP stay visible; picking or
  erasing a boundary takes its viewport along.
- XCLIP Clipdepth: a rejected front plane keeps the old front plane and
  clears the back plane.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/scene/selection.rs

@@ -121,6 +121,19 @@ impl Scene {
         handles: &[Handle],
     ) -> Vec<Handle> {
         let mut expanded = self.expanded_with_leaders(handles);
+        // A viewport's clip boundary goes with its viewport: picking the
+        // boundary selects both, erasing it erases the viewport.
+        let viewports: Vec<Handle> = expanded
+            .iter()
+            .filter_map(|h| self.document.get_entity(*h).map(|e| (*h, e)))
+            .flat_map(|(h, e)| {
+                e.common().reactors.iter().copied().filter(move |r| {
+                    matches!(self.document.get_entity(*r),
+                        Some(EntityType::Viewport(vp)) if vp.clip_boundary_handle == h)
+                })
+            })
+            .collect();
+        expanded.extend(viewports);
         expanded.sort_unstable_by_key(|handle| handle.value());
         expanded.dedup();
         expanded